The present invention relates generally to digital communication systems and, more particularly, to satellite communication systems in which information is transmitted from one point to another on or near the earth's surface, by way of one or more satellite-based space stations. The power flux density (PFD) of each uplink beam to a satellite and of each downlink beam from a satellite is limited to a prescribed maximum value by governmental regulation, specifically by the Federal Communications Commission (FCC) in the United States and by international and other agencies in other countries. The power flux density is defined as the measured power, in watts per square meter per hertz of spectral bandwidth. A related measure of radiated power density is the power spectral density (PSD), which is defined as the measured power per hertz of bandwidth.
For most efficient operation, most communication systems operate at at or close to the maximum PFD limit. When operating in this condition, the system signal-to-noise ratio (SNR) cannot be increased either by decreasing the signaling rate or increasing the total transmitted power without exceeding the PFD limit.
There is sometimes a need to compensate for attenuation by rain on the path from the satellite to the ground receiver. If the rain attenuation is uniform over the entire beam, the decreased SNR can be mitigated by reducing the signaling rate in proportion to the increase in attenuation, and the PFD at the surface of the earth will not exceed the PFD limit. If, however, the rain is localized so it affects only a portion of the beam, compensating for rain attenuation by decreasing the signaling rate will result in the PFD limit being exceeded in those regions unaffected by rain.
It will be appreciated from the foregoing that there is need for a technique that will allow reducing the data rate to compensate for rain attenuation without affecting the power flux density of the beam. The present invention is directed to this end.